1. Field of the Invention
The present invention pertains to a particular chemistry which provides advantages in the pattern etching a copper layer on the surface of a semiconductor device substrate. In particular, the etched portion of a feature surface is protected from reactive species during the etching of adjacent feature surfaces.
2. Brief Description of the Background Art
In the multi level metallization architecture used in present day semiconductor devices, aluminum is generally used as the material of construction for interconnect lines and contacts. Although aluminum offers a number of advantages in ease of fabrication, as integrated circuit designers focus on transistor gate velocity and interconnect line transmission time, it becomes apparent that copper is the material of choice for the next generation of interconnect lines and contacts. In particular, when the aluminum wire size becomes smaller than 0.5 xcexcm, the electromigration resistance and the stress migration resistance of aluminum becomes a problem area. In addition, when the feature size of an aluminum-based contact requires an aspect ratio of greater than 1:1, it is difficult to obtain planarization of the substrate during the application of the next insulating layer over the contact area of the substrate. Further, the resistivity of copper is about 1.4 xcexcxcexa9cm, which is only about half of the resistivity of aluminum.
There are two principal competing technologies under evaluation by material and process developers working to enable the use of copper. The first technology is known as damascene technology. In this technology, a typical process for producing a multilevel structure having feature sizes in the range of 0.5 micron (xcexcm) or less would include: blanket deposition of a dielectric material; patterning of the dielectric material to form openings; deposition of a diffusion barrier layer and, optionally, a wetting layer to line the openings; deposition of a copper layer onto the substrate in sufficient thickness to fill the openings; and removal of excessive conductive material from the substrate surface using chemical-mechanical polishing (CMP) techniques. The damascene process is described in detail by C. Steinbruchel in xe2x80x9cPatterning of copper for multilevel metallization: reactive ion etching and chemical-mechanical polishingxe2x80x9d, Applied Surface Science 91 (1995) 139-146.
The competing technology is one which involves the patterned etch of a copper layer. In this technology, a typical process would include deposition of a copper layer on a desired substrate (typically a dielectric material having a barrier layer on its surface); application of a patterned hard mask or photoresist over the copper layer; pattern etching of the copper layer using wet or dry etch techniques; and deposition of a dielectric material over the surface of the patterned copper layer, to provide isolation of conductive lines and contacts which comprise various integrated circuits. An advantage of the patterned etch process is that the copper layer can be applied using sputtering techniques well known in the art. The sputtering of copper provides a much higher deposition rate than the evaporation or CVD processes typically used in the damascene process, and provides a much cleaner, higher quality copper film than CVD. Further, it is easier to etch fine patterns into the copper surface and then deposit an insulating layer over these patterns than it is to get the barrier layer materials and the copper to flow into small feature openings in the top of a patterned insulating film.
Each of the above-described competing technologies has particular process problems which must be solved to arrive at a commercially feasible process for device fabrication. In the case of the damascene process, due to difficulties in the filling of device feature sizes of 0.25 xcexcm and smaller (and particularly those having an aspect ratio greater than one) on the surface of the dielectric layer, the method of choice for copper deposition is evaporation (which is particularly slow and expensive); or chemical vapor deposition, or CVD (which produces a copper layer containing undesirable contaminants and is also a relatively slow deposition process). Just recently, electroplating has been investigated as a method for copper deposition.
Regardless of the technique used to deposit copper, the CMP techniques used to remove excess copper from the dielectric surface after deposition create problems. Copper is a soft material which tends to smear across the underlying surface during polishing. xe2x80x9cDishingxe2x80x9d of the copper surface may occur during polishing. As a result of dishing, there is variation in the critical dimensions of conductive features. Particles from the slurry used during the chemical mechanical polishing process may become embedded in the surface of the copper and other materials surrounding the location of the copper lines and contacts. The chemicals present in the slurry may corrode the copper, leading to increased resistivity and possibly even corrosion through an entire wire line thickness. Despite the number of problems to be solved in the damascene process, this process is presently viewed in the industry as more likely to succeed than a patterned copper etch process for the following reasons.
The patterned etch process particularly exposes the copper to corrosion. Although it is possible to provide a protective layer over the etched copper which will protect the copper form oxidation and other forms of corrosion after pattern formation, it is critical to protect the copper during the etch process itself to prevent the accumulation of involatile corrosive compounds on the surface of the etched copper features. These involatile corrosive compounds cause continuing corrosion of the copper even after the application of a protective layer over the etched features.
Wet etch processes have been attempted; however, there is difficulty in controlling the etch profile of the features; in particular, when the thickness of the film being etched is comparable to the minimum pattern dimension, undercutting due to isotropic etching becomes intolerable. In addition, there is extreme corrosion of the copper during the etch process itself.
Plasma etch techniques provide an alternative. A useful plasma etch process should have the following characteristics: It should be highly selective against etching the mask layer material; it should be highly selective against etching the material under the film being etched; it should provide the desired feature profile (e.g. the sidewalls of the etched feature should have the desired specific angle); and the etch rate should be rapid, to maximize the throughput rate through the equipment.
Until very recently etch rates obtained by purely physical bombardment were typically about 300 xc3x85-500 xc3x85 per minute or less, as described by Schwartz and Schaible, J. Electrochem. Soc., Vol. 130, No. 8, p. 1777 (1983) and by H. Miyazaki et al., J. Vac. Sci. Technol. B 15(2) p.239 (1997), respectively. Recently, applicants have been able to improve on the etch rates achievable by purely physical bombardment so that etch rates as high as 5,000 xc3x85 per minute can be achieved. Further, the selectivity between copper and materials commonly used as barrier layers, insulating layers and patterning masks is more than satisfactory. This technology is disclosed in detail in pending U.S. Pat. No. 6,010,603, filed Jul. 9, 1997. However, etch rate and selectivity must be accompanied by the ability to etch a pattern having the desired cross-sectional profile. To improve etch profile, it is necessary to use a limited amount of chemical reactants during the etch process.
The chemical reactants must be very carefully selected to react with the copper and create volatile species which can then be removed by application of vacuum to the process chamber. However, when such chemical reactants are used, corrosion is a major problem during the fabrication, as copper does not form any self passivating layer like aluminum does. In particular, oxidation of copper increases resistivity; further, in the case of copper interconnect lines, the whole wire line may corrode all the way through, resulting in device failure. As described in U.S. Pat. No. 6,010,603, referenced above, it is possible to use a limited concentration of particular halogen-based reactants in combination with physical bombardment, when physical bombardment is the controlling etch mechanism and avoid corrosion of the copper by the reactive species used to assist in the etch process.
There are some etch profiles for which etching in the physical bombardment regime does not provide the best result. In addition, applicants have discovered that it is possible to obtain etch rates which are higher than those obtained to date in the physical bombardment regime and still avoid corrosion of the etched copper. Typically, a chlorine-comprising gas is used in the reactive ion etch processing of the copper. Although the chlorine provides acceptable etch rates, it causes the copper to corrode rapidly. The chlorine reacts very fast, but produces reaction by-products which are not volatile. These byproducts remain on the copper surface, causing corrosion over the entire etched surface. The byproducts can be made volatile subsequent to the etch step by treatment with chemical species which create a volatile reaction product, but by this time the corrosion is already extensive.
An example of a treatment to remove chlorides and fluorides remaining after the etch of a conductive layer is provided in U.S. Pat. No. 4,668,335 to Mockler et al., issued May 26, 1987. In Mockler et al., the workpiece (wafer) is immersed in a strong acid solution, followed by a weak base solution after the etch of an aluminum-copper alloy, to remove residual chlorides and fluorides remaining on the surface after etching. Another example is provided in U.S. Pat. No. 5,200,031 to Latchford et al., issued Apr. 6, 1993. In Latchford et al, a process is described for removing a photoresist remaining after one or more metal etch steps which also removes or inactivates chlorine-containing residues, to inhibit corrosion of remaining metal for at least 24 hours. Specifically, NH3 gas is flowed through a microwave plasma generator into a stripping chamber containing the workpiece, followed by O2 gas (and optionally NH3 gas), while maintaining a plasma in the plasma generator.
Attempts have been made to reduce the corrosion by introducing additional gases during the etch process (which can react with the corrosion causing etch byproducts as they are formed). In addition, gaseous compounds which can react to form a protective film over the sidewalls of etched features as they are formed have been added during the etching process and after the etch process. However, residual corrosion continues to be a problem and the protective film, while protecting from future contact with corrosive species, may trap corrosive species already present on the feature surface.
An example of the formation of a passivating film on pattern sidewalls is presented by J. Torres in xe2x80x9cAdvanced copper interconnections for silicon CMOS technologiesxe2x80x9d, Applied Surface Science, 91 (1995) 112-123. Other examples are provided by Igarashi et al. in: xe2x80x9cHigh Reliability Copper Interconnects through Dry Etching Processxe2x80x9d, Extended Abstracts of the 1994 International Conference on Solid State Devices and Materials, Yokohama, 1994, pp.943-945; in xe2x80x9cThermal Stability of Interconnect of TiN/Cu/TiN Multilayered Structurexe2x80x9d, Jpn. J. Appl. Phys. Vol. 33 (1994) Pt. 1, No. 1B; and, in xe2x80x9cDry Etching Technique for Subquarter-Micron Copper Interconnectsxe2x80x9d, J. Electrochem. Soc., Vol. 142, No. 3, March 1995. In this 1995 article, Yasushi Igarashi et al. show photomicrographs of cross-sectional views of the subquarter-micron etched features. In reviewing the article, applicants noticed that although the exterior walls of the feature appear to be solid, there appears to be interior hollow areas within the feature where the copper line has been eroded away. Applicants subsequently reproduced this effect, demonstrated by the comparitive example (Example 3) presented subsequently herein. Apparently, reactive chlorine species are trapped interior to the passivating film formed on the wall and these species react with and erode the copper beneath the passivating film.
Passivating films are used to protect the walls of forming features during the etching of aluminum. Such films are generally used to protect the walls of etched features from further etching by incident reactive species during continued vertical etching of the feature through a mask. Typically the protective film comprises an oxide or a nitride or a polymeric material, or a combination thereof. In the case of aluminum, aluminum oxide forms a cohesive, continuous protective film very rapidly. This rapid formation of a continuous protective film protects the interior of the etched feature from exposure to significant amounts of the reactive species which could cause corrosion interior to the etched wall. However, in the case of copper, there is no similar rapidly-formed film which prevents reactive species from reaching the copper surface and being trapped there by a slowly-formed xe2x80x9cpassivatingxe2x80x9d film. It appears that the passivating films of the kind described by Igarashi et al. in their March 1995 article trap reactive species inside the feature walls and these reactive species corrode away the copper interior to the feature walls.
If the patterned etch technique is to be used for fabrication of semiconductor devices having copper interconnects, contacts, and conductive features in general, it is necessary to find an etch method which does not create immediate corrosion or a source of future corrosion.
In addition to controlling corrosion, it is necessary to control the profile of the etched pattern. An example of a technique used for obtaining a high etch rate and highly directional reactive etching of patterned copper films copper is described by Ohno et al in xe2x80x9cReactive Ion Etching of Copper Films in a SiCl4, N2, Cl2, and NH3 Mixturexe2x80x9d, J. Electrochem. Soc., Vol. 143, No. 12, December 1996. In particular, the etching rate of copper is increased by increasing the Cl2 flow rate at temperatures higher than 280xc2x0 C. However, the addition of Cl2 is said to cause undesirable side etching of the Cu patterns. NH3 is added to the gas mixture to form a protective film that prevents side wall etching. The etch gas mixture which originally contained SiCl4 and N2 was modified to contain SiCl4, N2, Cl4, and NH3.
Thus, protective films formed during etching are used by some practitioners skilled in the art to reducing corrosion (as described above) and by others for controlling the directional etching of the pattern surface. In either case, although the formation of such a protective film may work well for aluminum etching, it may be harmful in the case of copper etching for the reasons previously described.
Toshiharu Yanagida, in Japanese Patent Application No.4-96036, published Oct. 22, 1993, describes a method of dry etching of a copper material in the temperature range at which a polymeric resist mask can be used (below about 200xc2x0 C.). Etching using a polymeric resist mask is said to be preferable so that the presence of oxygen (present in a silicon oxide hard masking material which can withstand higher temperatures) can be avoided. The oxide causes harmful corrosion of the copper, producing copper oxides which increase the resistivity of etched copper features. In particular, the Yanagida reference describes the use of hydrogen iodide (HI) gas and combinations of HI gas with chloride and/or fluoride compounds to achieve etching at substrate temperatures below about 200xc2x0 C.
B. J. Howard and C. Steinbuchel, in their article entitled xe2x80x9cReactive ion etching of copper with BCl3 and SiCl4: Plasma diagnostics and patterningxe2x80x9d, J. Vac. Sci. Technol. A 12(4), July/August, 1994, describe the etching of copper film using a plasma source gas mixture of BCl3/N2 or BCl3 /Ar. Gas mixtures with N2 are said to provide considerably higher etch rates than mixtures with Ar. This is attributed to nitrogen scavenging the available B to produce BN, which prevents the recombination of Cl with B, thereby increasing the amount of free Cl However, the authors acknowledged that there are patterning problems when BCl3/N2 is used, despite acceptable etch rates. When we tried to reproduce the results of Howard and Steinbuchel, we discovered that the etched sidewalls of copper lines in a pattern of lines and spaces were deeply pitted, i.e. there were large pockets of void spaces where the etchant had removed pockets of copper.
FIGS. 5A and 5B illustrate the kind of corrosion which typically is experienced during the reactive ion etching of copper. The pattern etched was one of lines and spaces, wherein the lines and spaces were approximately 0.5 xcexcm in width looking at a cross-sectional profile of the pattern. The details of the preparation of the etched substrates shown in FIGS. 5A and 5B will be discussed in detail subsequently herein, for comparitive purposes. For now, the important features to note are that the copper lines 510 which appear to be solid looking at the exterior walls 516 are actually hollow in the interior, where the copper 520 remaining after etching is surrounded by vacant space 522. The vacant space is created by the harmful copper reactions we are calling xe2x80x9ccorrosionxe2x80x9d. Corrosion is caused when the copper reacts with oxygen or with other reactants present in the process vessel to produce undesirable by-products. Corrosion also includes reaction with halogens which are typically used as etchant reactants, but the reaction occurs at an undesired rate so that the desired etched feature profile cannot be obtained or the surface of the etched feature is badly pitted. FIGS. 5A and 5B are representative of etched copper lines and spaces, where the etched copper lines exhibit interior corrosion and poor exterior profile.
We have discovered basic chemistries and process parameters which make it possible to etch micron and submicron sized copper features on a semiconductor substrate while maintaining the integrity of the etched copper feature.
We have discovered that copper can be pattern etched in a manner which provides the desired feature dimension and integrity, at acceptable rates, and with selectivity over adjacent materials. To provide for feature integrity, the portion of the copper feature surface which has been etched to the desired dimensions and shape must be protected during the etching of adjacent feature surfaces. This is particularly important for feature sizes less than about 0.5 xcexcm, where presence of even a limited amount of a corrosive agent can eat away a large portion of the feature. The copper feature integrity is protected by several different mechanisms: 1) The reactive etchant species are designed to be only moderately aggressive without ion bombardment, so that an acceptable etch rate is achieved without loss of control over the feature profile or the etch surface; 2) Hydrogen is applied over the etch surface so that it is absorbed onto the etch surface, where it acts as a boundary which must be crossed by the reactive species and a chemical modulator for the reactive species; and 3) Process variables are adjusted so that byproducts from the etch reaction are rendered more volatile and easily removable from the etch surface.
Preferred chlorine-comprising etchant species include HCl, or HCl*, or HCl+, or HClxe2x88x92, or Cl, or Cl*, or Cl+, or Clxe2x88x92, or combinations thereof, as opposed to Cl2, or Cl2*, or Cl2+, or Cl2xe2x88x92. In an inductively coupled plasma etch chamber, we have observed that the preferred chlorine reactive species are generated when the chlorine is dissociated from compounds rather than furnished as Cl2 gas.
The hydrogen which is applied over the etch surface is particularly important, as it is available to react with species which would otherwise penetrate that exterior surface and react with the copper interior to that surface. Sufficient hydrogen must be applied to the exterior surface of the etched portion of the copper feature to prevent incident reactive species present due to etching of adjacent feature surfaces from penetrating the previously etched feature exterior surface.
Although any plasma feed gas component comprising hydrogen, which is capable of generating sufficient amounts of hydrogen, may be used, the most preferred embodiment of the invention provides for the use of a component which contains both hydrogen and halogen. The etch chemistry for one of the more preferred embodiments utilizes hydrogen chloride (HCl) and/or hydrogen bromide (HBr), as the principal source of the reactive species for etching a copper surface. Dissociation of the HCl and/or HBr provides large amounts of hydrogen for absorption onto the etched copper surfaces, thereby preventing penetration by reactive species into the interior of adjacent the etched surfaces. Further, etch reaction with HCl species is less than with Cl2 species, and the reaction product and by products are more volatile when HCl etch species are used.
Additional hydrogen gas may be added to the plasma feed gas which comprises the HCl and/or HBr when the reactive species density in the etch process chamber is particularly high. The hydrogen-releasing, halogen-comprising plasma feed gas component may be used as an additive (producing less than 40% of the plasma-generated reactive species) in combination with other plasma etching species.
When HCl and/or HBr is used as the principal source of reactive species for the copper etching, the HCl or HBr accounts for at least 40%, and more preferably at least 50%, of the reactive species generated by the plasma. Most preferably, HCl or HBr accounts for at least 80% of such reactive species. Other reactive species may be used for purposes of improving the etched feature profile, improving etch selectivity, or reducing microloading effects. The species added for these purposes during etching of the copper feature preferably make up 30% or less, or more preferably make up 10% or less of the plasma-generated reactive species.
In particular, the preferred method for the etching of a copper surface to provide a patterned semiconductor device feature, includes the following steps:
a) supplying to a plasma etch process chamber a plasma feed gas which serves as a source for a dissociated halogen species and as a source for hydrogen, wherein the amount of hydrogen generated is sufficient to act as a boundary layer and a chemical modulator for the dissociated halogen species as the species react with a copper surface to be etched;
b) maintaining the temperature of the copper surface to be etched at a temperature sufficient to provide for advantageous volatility of halogen-containing etch reaction products and byproducts; and
c) etching said copper surface to provide the desired patterned feature.
In one of the more preferred embodiments, the plasma feed gas includes HCl, or HBr, or a combination thereof, in an amount which provides plasma-generated dissociated chlorine or bromine or a combination thereof and dissociated hydrogen in an amount sufficient to act as a boundary layer and chemical modulator for incident halogen-comprising species which strike a portion of a copper feature which has been etched to the desired dimensions. Gaseous hydrogen may be added to the plasma feed gas to adjust the ratio of the amount of hydrogen to the amount of halogen-comprising species, as needed to assist in obtaining the desired etch profile, etch selectivity or to reduce microloading effects.
In addition to the addition of gaseous hydrogen, other species-generating gases may be added for purposes of passivating various surfaces and reacting with potentially corrosive species on the semiconductor substrate, as well as assisting in profile control, or in etch selectivity or to reduce microloading effects. Other additives may be, for example and not by way of limitation, nitrogen, fluorine-comprising molecules (which are particularly useful in etching barrier layers adjacent the copper layer which is to be patterned), chlorine-comprising molecules which do not contain hydrogen, inorganic hydrogen containing molecules, and hydrocarbon molecules. Specific examples of such molecules include, but are not limited to CxHy, BCl3, SiCl4, CCl4, CH3F, CHF3, N2 NH3, NH2OH, HI, H3As, H2S, H2Te, H4P2, and H3P, wherein x ranges from 1 to about 4 and y ranges from 2 to about 10.
Plasma feed gases may include additional inert (non-reactive with copper) gases such as argon, helium, or xenon, to enhance the ionization, or dissociation, or to dilute the reactive species. Further, such inert gases may be used as plasma source gases for purposes of heating up a semiconductor substrate during and/or prior to a copper etch.
In a particularly preferred embodiment of the present invention, copper is patterned on a substrate surface for use in semiconductor fabrication, wherein the method steps comprise:
a) supplying a plasma feed gas to a plasma etch process chamber, or other controlled environment for the production of a plasma, wherein said feed gas includes HCl, or HBr, or a combination thereof, wherein the amount of HCl or HBr is sufficient that at least 40%, and preferably at least 50%, of the total reactive species present in the plasma are supplied by the HCl or HBr or combination thereof; and
b) using a plasma created from the plasma feed gas in a manner which provides a reactive species density sufficient to enable a copper etch rate of at least about 2,000 xc3x85 per minute.
Hydrogen gas or a hydrogen-containing compound may be added to the feed gas in an amount which depends on the density of reactive species present at the surface of the copper during etching. Hydrogen gas may be added to the plasma feed gas for only a portion of a copper feature etch time period.
As previously mentioned, other gases capable of generating reactive species for purposes of surface passivation or as an oxygen xe2x80x9cgetterxe2x80x9d, such as N2 or BCl3, respectively, may be used in the plasma feed gas. In addition, inert gases such as argon may be used.
The critical feature is the availability of hydrogen at the feature surface during the etching process. The use of HCl or HBr as the primary source of the copper etchant reactive species provides for the availability of dissociated, reactive hydrogen, as the hydrogen is released upon creation of the plasma and is adsorbed on or absorbed near the copper surface during etching, where it buffers the reaction of the chlorine or bromine species with the copper surface which is being etched. This protects the interior of the copper feature from subsequent corrosion while permitting etch rates for adjacent copper surfaces of at least 2,000 xc3x85 per minute. To enhance the amount of dissociated hydrogen available, hydrogen may be generated from hydrogen-containing compounds.
Another important feature of the use of HCl and HBr to generate reactive etchant species is that the species generated are less aggressive than those generated from Cl2, so an acceptable etch rate is achieved and the volatility of the etch products and byproducts is greater, so that the corrosive activity at the copper surface is reduced.
When the etch process is carried out typically oxygen-comprising species are generated from a silicon oxide hard mask or a photoresist, or from an insulating layer present on the substrate. Under these circumstances, it is advantageous to add boron trichloride (BCl3) or an equivalent oxygen scavenger.
Another preferred embodiment relates to the use of a plasma source gas including a halogen-comprising primary etchant species (preferably generated from a halogen-comprising compound) in combination with a hydrogen-generating source. Preferably the halogen-comprising primary etchant species is generated from a low molecular weight compound such as, and not by way of limitation, HCl, HBr, BCl3, CHF3, SiCl4, CCl4, and combinations thereof. The most preferred compound for this embodiment is BCl3. The hydrogen-generating source may include hydrogen gas, but always includes at least one hydrogen-generating compound, preferably a low molecular weight hydrocarbon. Examples of the hydrogen-generating compound include, for example, but not by way of limitation, CxHy, CH3F, CHF3, NH3, NH2OH, HI, H3As, H2S, H2Te, H4P2, and H3P, where x ranges from 1 to about 4 and y ranges from 2 to about 10.
Diluent gases such as argon, helium, krypton, and xenon, and complexing gases such as nitrogen (which promote the presence of reactive halogen species) may be used in combination with the primary etchant source compound and the hydrogen-generating source.
With regard to all of the embodiments of the invention, whether the primary etchant is HBr, HCl, BCl3, or one of the other named sources for a halogen-comprising species, when other process conditions and materials (such as plasma source power, or substrate bias power, or substrate temperature, or plasma source gas flow rate), which are necessary to provide the desired etch rate or etched pattern profile, cause the surface of the etched profile to exhibit pits or pockets, an increase in the process chamber pressure can be used to reduce the damage to the etched surface. In particular, rather than operating at a process chamber pressure in the range from about 0.1 mT to about 20 mT which is typically used in an inductively coupled plasma chamber, the process chamber pressure is increased to be in the range from about 20 mT up to about 200 mT. In a capacitively coupled plasma chamber the higher pressure regime ranges from about 100 mT up to 5 Torr. As the chamber pressure is increased, copper etch rate is increased and the electron temperature of the etchant species decreases, whereby plasma damage to the device structures (such as transistors) beneath the copper etch surface is reduced.